While experimental analyses of steady rectilinear locomotion in fishes are
common, unsteady movement involving time-dependent variation in heading, sp
eed and acceleration probably accounts for the greatest portion of the loco
motor time budget. Turning maneuvers, in particular, are key elements of th
e unsteady locomotor repertoire of fishes and, by many species, are accompl
ished by generating asymmetrical forces with the pectoral fins. The develop
ment of such left-right asymmetries in force production is a critical and a
s yet unstudied aspect of aquatic locomotor dynamics. In this paper, we mea
sure the fluid forces exerted by the left and right pectoral fins of bluegi
ll sunfish (Lepomis macrochirus) during turning using digital particle imag
e velocimetry (DPIV), DPIV allowed quantification of water velocity fields,
and hence momentum, in the wake of the pectoral fins as sunfish executed t
urns; forces exerted during turning were compared with those generated by t
he immediately preceding fin beats during steady swimming. Sunfish generate
the forces required for turning by modulating two variables: wake momentum
and pectoral fin stroke timing. Fins on opposite sides of the fish play fu
nctionally distinct roles during turning maneuvers. The fin nearer the stim
ulus inducing the turn (i.e. the strong side fin) generates a laterally ori
ented vortex ring with a strong central jet whose associated lateral force
is four times greater than that produced during steady swimming. Little pos
terior (thrust) force is generated by the strong-side fin, and this fin the
refore acts to rotate the body away from the source of the stimulus. The co
ntralateral (weak-side) fin generates a posteriorly oriented vortex ring wi
th a thrust force nine times that produced by the fin during steady swimmin
g. Minimal lateral force is exerted by the weak-side fin, and this fin ther
efore acts primarily to translate the body linearly away from the stimulus.
Turning with the paired fins is not simply steady swimming performed unila
terally. Instead, turning involves asymmetrical hn movements and fluid forc
es that are distinct in both direction and magnitude from those used to swi
m forward at constant speed. These data reflect the plasticity of the teleo
st pectoral fin in performing a wide range of steady and unsteady locomotor
tasks.